Wheatstone Bridges

Wheatstone bridges are circuits used to measure resistances:

In this example, the resistances are R₁, R₂, R₃, R₄ and R_m.  Let I₁, I₂, I₃, I₄ and I_m denote the corresponding currents.  When I_m = 0:

I₁ = I₃
I₂ = I₄
I₁R₁ = I₂R₂
I₃R₃ = I₄R₄

which implies R₄ = R₂R₃ / R₁.